Fluoropolymers are primarily produced via heterogeneous polymerization reactions, including suspension, emulsion and microemulsion systems. Generally, each of these reactions requires at least one monomer and a radical initiator in a suitable reaction medium. In addition, emulsion polymerizations of halogenated monomers generally require a surfactant capable of emulsifying both the reactants and the reaction products for the duration of the polymerization reaction. The surfactant of choice in the synthesis of fluoropolymers is generally a perfluoroalkyl surfactant. The most common perfluoroalkyl surfactant in the production of halogenated polymers is ammonium perfluorooctanoate (APFO). Non-fluorinated surfactants typically are not used, because they have the disadvantages of inhibiting the reaction, and affecting the molecular weight and other properties of the product, such as color.
The emulsion polymerization of vinylidene fluoride (VF2) using a fluorinated surfactant and isopropyl peroxydicarbonate (IPP) as a free-radical initiator, is taught in U.S. Pat. No. 3,475,396. This polymerization process has since been refined to provide polymers having various improved properties. See, U.S. Pat. Nos. 3,857,827, 4,076,929, 4,360,652, 4,569,978, 6,187,885. The emulsion polymerization of VF2 using a partially fluorinated surfactant and an inorganic peroxide initiator is taught in U.S. Pat. No. 4,025,709.
The common use of fluorinated surfactants such as APFO in the emulsion polymerization of fluoromonomers is dictated by the difference in the electronic environment in a fluoromonomer versus that in a nonfluorinated monomer. See, Ameduri et al., “Copolymerization of fluoromonomers: recent developments and future trends”, Journal of Fluorine Chemistry, vol. 104, pp. 53-62, 2000; the entire contents of which is incorporated herein by reference.
The presence of hydrocarbon compounds, including non-fluorinated hydrocarbon surfactants, in radical polymerization reactions of fluoromonomers such as VF2, leads to chain-transfer. Examples of such non-fluorinated surfactants include sodium dodecyl sulfate (SDS), sodium dodecyl benzenesulfonate, and polyoxyethylene alkyl phenyl ether.
The chain-transfer associated with non-fluorinated hydrocarbon surfactants is due to the ease of atom transfer from the hydrocarbon to the forming polymer. For example, measurement of transfer constants from alcohols to polymerizing tetrafluoroethylene has been shown to be two to three orders of magnitude higher than transfer constants to polymerizing methyl methacrylate under otherwise identical reaction conditions. See, Berger et al., “Transfer Constants to Solvents and Additives,” Polymer Handbook, 3rd Edition, part E, Section II, pp. 106, 110, 111, 121, John Wiley & Sons, New York, 1989. The higher transfer constants for fluoropolymers result in decreased process rates and prevention of the generation of desired high-molecular-weight polymers. For this reason, fluorinated surfactants are presently the surfactant of choice in emulsion polymerization of fluoromonomers.
The use of perfluorinated surfactants has recently become the focus of increasing scrutiny due to safety and environmental concerns. A primary property of perfluoroalkyl surfactants that underlies their utility in polymerization reactions is their stability to the reaction conditions. Because of their resistance to chemical degradation, fluoroalkyl surfactants have the potential to accumulate in the environment and in organisms.
Several different approaches have attempted to reduce or eliminate perfluoroalkyl surfactants as components in the polymerization of fluoromonomers.
Some emulsion polymerization processes have been demonstrated which employ partially fluorinated surfactants instead of perfluorinated surfactants. See, U.S. Pat. Nos. 4,524,197, 5,763,552. Another attempt to reduce the amount of perfluoroalkyl surfactant in heterogeneous polymerization has involved utilization of fluorinated surfactant—non-fluorinated hydrocarbon surfactant combinations. However, this modification served to substantially lower the rate of the reaction. See, WO 95-08598A, the entire disclosure of which is incorporated herein by reference.
In another modification, the perfluoroalkyl surfactant was eliminated by employing a surfactant-free polymerization process. The process relies on the use of metal salts as promoters. See, WO 97 17381 and J. Appl. Polym. Sci., 2211, 70, 1998. The metal salts can, however, contaminate the product.
An alternative approach to stabilizing emulsion polymerizations involves the use of copolymerizable surfactants. A copolymerizable surfactant serves as both a comonomer and as a surfactant. Copolymerizable surfactants are particularly useful in preparing coatings. Examples of copolymerizable surfactants in polymerization of fluoromonomers include: Fluorinated vinyl ether sulfonates or carboxylates in making vinylidene fluoride polymer and copolymers (Tsuda et at., patent publication WO 96/06887 A1); unsaturated polyether/anionic sulfonate, carboxylate, or phosphate, and unsaturated polyether in making fluoropolymers (Naniwa et al., Japan published unexamined patent application laid-open patent HEI{SEI} 9-316144, 9 Dec. 1997); and unsaturated polyether in making chlorotrifluoroethylene copolymers (Kodama, Shun-ichi et al., Japan, published unexamined patent application laid-open patent HEI{SEI} 9-59560, 4 Mar. 1997).
The sodium salt of 2-hydroxy-3-(2-propenyloxy)-1-propanesulfonic acid corresponds to Chemical Abstract (CA) number [52556-42-0]. The Chemical Abstracts name for CA[52556-42-0] is 1-propanesulfonic acid, 2-hydroxy-3-(2-propenyloxy)-, monosodium salt. The chemical name generally employed for the free acid is 3-allyloxy-2-hydroxy-1-propanesulfonic acid. See, Aldrich Handbook of Fine Chemicals and Laboratory Equipment, 2000-2001, page 50, catalog no. 40,942-1, referencing a 40% aqueous solution of 3-allyloxy-2-hydroxy-1-propanesulfonic acid, sodium salt.
The sodium salt of 3-allyloxy-2-hydroxy-1-propanesulfonic acid is described in a product bulletin published by Rhodia (sodium 3-allyloxy-2-hydroxypropyl sulfonate), as a copolymerizable surfactant recommended for emulsion polymerizations of vinyl acetate, n-butyl acrylate, styrene, vinyl chloride and chloroprene. The product described in the Rhodia bulletin is supplied as a 40% solution in water, and is described as a clear, clean liquid with a specific gravity (25°/25°) of 1.17, a Brookfield viscosity (cps) of 10, and a density (pounds/gallon) of 9.75 (0.97282 g/mL). Use of this polymerizable surfactant for fluoropolymer emulsion polymerization is not disclosed.
New processes for polymerization of fluoromonomers are needed that can utilize non-fluorinated surfactants, thereby reducing or eliminating the required amounts of perfluoroalkyl surfactants.